KABUL, Afghanistan — Insurgents in eastern Afghanistan opened fire yesterday on patrolling U.S. forces, setting off a firefight that left four attackers dead, a military spokesman said. No U.S. casualties were reported.

The U.S. troops came under attack before dawn near Shkin, a volatile town in Paktika province near the Pakistan border, the spokesman, Col. Rodney Davis, said from Bagram Air Base.

U.S. soldiers fired four artillery shells along with several illumination rounds during a three-hour exchange, Col. Davis said.

After an initial gunfight, “coalition forces then conducted a search of the contact site and a nearby compound,” he said. “They found three enemy killed in action and engaged a fourth who attempted to throw a hand grenade at coalition forces, killing him.”

The size of the insurgent group was not determined. They were armed with assault rifles and rocket-propelled grenade launchers, Col. Davis said.

The U.S. patrol was searching out guerrilla fighters, he said.

In a separate incident Monday, also near Shkin, unidentified attackers fired four rocket-propelled grenades at an Afghan military checkpoint on the Pakistan border. No casualties nor damage was reported.

Meanwhile, near Asadabad, another town to the northwest in eastern Kunar province, U.S. special operations forces recovered three Blowpipe optically guided, shoulder-launched surface-to-air missile systems. It was not clear to whom they belonged or where exactly they were found.

Insurgents have been stepping up attacks in the south and east of the country since the start of spring.

On Saturday, a suicide bomber killed four German peacekeepers and wounded 19 when he drove an explosives-laden car alongside a bus carrying the peacekeepers. One Afghan was also killed and as many as 10 wounded in the same attack.

About 11,500 coalition troops are in Afghanistan conducting operations to root out small groups of Taliban and al Qaeda terrorists and their allies, who authorities say are using bases in Pakistan to launch cross-border attacks. The Taliban was overthrown in a U.S.-led war in 2001.

In Washington a senior Pentagon official voiced concern yesterday about a “new pattern” of terror attacks on international troops and aid workers in Afghanistan.

Mr. Collins, in a breakfast meeting with reporters, added, “On the one hand, [there is] concern about these new tactics. On the other hand, I have to say that there is undoubtedly more stability in many areas of Afghanistan than there has been for years.”

“I am very bullish on Afghanistan,” added Mr. Collins, who is in charge of “force stability” at the Defense Department.

He sees no sign that a growing number of deaths of U.S. troops in attacks in Iraq and the killing of peacekeepers in Afghanistan will dampen the willingness of nations to provide military peacekeepers.